No, do not keep the old tracks. Several aspects to this question. First, there must definitely be wear and tear to your 1990 garage door tracks. They could be slightly bent, misaligned, twisted. Second, your older tracks are likely chain tracks, while you could get more modern tracks that have nylon rollers which are much, much quieter. Third, AFAIK you need to match the tracks/rails to the doors. Your new doors are unlikely to be of the same materials and weight as your 30+-year-old doors.

I had thought that every new garage door that is manufactured comes with its own set of tracks, but I could be wrong about that. Maybe someone on this page knows.

You don't HAVE TO change the track as long as the new door is compatible with the existing track design. But to stay on a safer side it's advisable to replace the track as well.

Garage door rollers are a wear item. Easily replaceable. I switched out old noisy steel rollers for nylon ones several years ago. Ordered the new rollers from the jungle river store and installed them in way less than a hour.

What is a 'chain track'? Garage door openers are either chain driven or belt driven but, AFAIK, standard residential overhead garage door tracks are just that, 'standard'. The only thing 'custom' is the install. Garage door opener hardware is an add on that uses the existing roller tracks & rollers anyway.

1990 tracks might actually be made from steel made in the USA and, like the installer told you, likely better quality than new ones. I would listen to the guy who has 40 years of experience. If he does your install and encounters any parts that need replacing, he will know what to do.

Does 4-5K really seem outlandish to you? I would normally replace the tracks. I don't know that steel has gotten worse over the last 20 years, although maybe his point is the mil thickness on the tracks.